Photoresponsive Zn2+-specific metallohydrogels coassembled from imidazole containing phenylalanine and arylazopyrazole derivatives

Article abstract of DOI:10.1039/d0dt01809k

Stimuli-responsive supramolecular gels and metallogels have been widely explored in the past decade, but the fabrication of metallogels with reversible photoresponsive properties remains largely unexplored. In this study, we report the construction of pho

Full text of DOI:10.1039/d0dt01809k

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Photoresponsive Zn2+-specific metallohydrogels
coassembled from imidazole containing phenyl-
alanine and arylazopyrazole derivatives†
Cite this: DOI: 10.1039/d0dt01809k
Ashanti Sallee and Kesete Ghebreyessus
*
Stimuli-responsive supramolecular gels and metallogels have been widely explored in the past decade,
but the fabrication of metallogels with reversible photoresponsive properties remains largely unexplored.
In this study, we report the construction of photoresponsive hybrid zinc-based metallohydrogel systems
coassembled from an imidazole functionalized phenylalanine derivative gelator (ImF) and carboxylic acid
functionalized arylazopyrazole (AzoPz) molecular photoswitches in the presence of Zn2+ ions. Unlike tra-
ditional covalent conjugation, noncovalent introduction of small molecular switches into the gel matrix
provides a convenient route to generate photoresponsive functional materials with tunable properties and
expands the scope of optically controlled molecular self-assemblies. It has been found that the carboxylic
acid functionalized AzoPz derivatives alone or mixed with the ImF moiety could not self-assemble to
form any gels. However, in the presence of Zn2+ ions they readily formed the coassembled hybrid metal-
logels in an alkaline aqueous solution with various morphologies. These results suggest that the gelation
process was triggered by the Zn2+ ions. In addition, the ImF gelator shows specific response to Zn2+ ions
only. The presence of the AzoPz moiety in the gel matrix makes the metallogel coassemblies photore-
sponsive and the reversible gel-to-sol phase transition was studied by UV-vis spectroscopy. The gels
showed a slow reversible light-induced gel-to-sol phase transition under UV (λ = 365 nm) and then sol-
to-gel transition by green light (λ = 530) irradiation resulting in the reformation of the original gel state.
The morphology and viscoelastic properties of the fibrillar opaque metallogels have been characterized
by transmission electron microscopy (TEM) and rheological measurement, respectively.
Received 19th May 2020,
Accepted 6th July 2020
DOI: 10.1039/d0dt01809k
rsc.li/dalton
nation.24 The incorporation of metal-ions into the supramole-
cular assembly can enrich the properties of these soft
Introduction
Stimuli-responsive supramolecular gels formed from low-mole- materials with metal specific functionality such as optical,
cular-weight gelators (LMWGs) have emerged as a promising magnetic, electronic, luminescence, and enhanced catalytic
class of soft materials, in part, due to their potential appli- properties.25 To date, a variety of metallogels based on metal
cations in chemical sensing,1–5 tissue engineering,6–8 switch- ion coordination with various organic ligands, amino-acids
able catalysis,9–11 environmental remediation,12–14 drug and peptides have been reported.24–29 The dynamic and revers-
delivery15–18 and other materials science fields. The main ible nature of the metal–ligand coordination make the metallo-
driving forces for LMWG molecules to self-assemble into nano- gels responsive to a variety of stimuli such as pH, temperature,
scale structures are generally weak noncovalent interactions chemical species and light.24e,f,25 Among these, metallogels
involving hydrogen bonding, π–π stacking, hydrophobic inter- that incorporate photoresponsive functionalities are particu-
actions and coordination interactions19–23 Recently, metal- larly attractive owing to their potential to design adaptive
induced gelation of LMWGs has also been recognized as a hybrid soft materials with tunable gelation properties by light-
promising strategy for the fabrication of gels with multiple irradiation.
stimuli sensitivity by tuning the dynamic metal–ligand coordi-
Normally, the construction of light-responsive metal–ligand
supramolecular assemblies is achieved through the incorpor-
ation of photochromic compounds in the structure of their
ligands. Irradiation with a specific wavelength of light enables
alteration of the conformation of the photochromic ligand,
which in turn may result in a structural reconfiguration of the
metallogel assembly.26 However, the fabrication of photore-
Department of Chemistry and Biochemistry, Hampton University, Hampton, VA,
23668, USA. E-mail: Kesete.Ghebreyessus@hamptonu.edu; Tel: +1-757-727-5475
†Electronic supplementary information (ESI) available: Synthetic route, 1H and
13C NMR spectra, UV-Vis spectra, optical pictures, IR spectra and mass spectra.
See DOI: 10.1039/D0DT01809K
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sponsive metal–ligand assemblies that incorporate photochro- of new photoswitchable metallohydrogels that would enable
mic ligands in their supramolecular structure is not straight- the reversible photomodulation of the gels’ physicochemical
forward. The design of suitable photochromic ligands that properties.
coordinate with metal ions and self-assemble to form gels
In this study, we report the fabrication of a photoresponsive
often requires substantial time and complicated synthetic zinc-based metallogel system coassembled from an imidazole-
efforts. An alternative but relatively unexplored approach to substituted phenylalanine derivative LMWG ligand (ImF) and
make metal–ligand assemblies light-responsive is through the carboxylic acid functionalized arylazopyrazole (AzoPz) small
noncovalent introduction of photoswitchable moieties to form molecular switches in the presence of Zn2+ ions in alkaline
metallogels via controllable coassembly.26b This approach may aqueous solution at pH ∼ 11.00 (Fig. 1). Vortex treatment of
provide a simple and efficient method to introduce functional the initial turbid mixtures of the compounds and the presence
moieties to the molecular structure and enables reversible of Zn2+ ions were found to be critical factors for the supramo-
photo-modulation of nonphotoresponsive supramolecular lecular assembly and formation of stable metallogels. Out of
assemblies. Most of the gels described in the literature thus the different metal salts that were tested, stable metallogels
far are obtained from single-component LMWGs in aqueous are selectively formed in the presence of zinc salts only. It is to
or organic medium; however, some are formed from multi- be noted that although the construction of La3+-based metallo-
component
coassemblies
using
the
supramolecular hydrogels using the ImF gelator has been recently reported,27c
approach.6,30–38 Compared to the single-component gels, the the design of light-responsive metallogels utilizing the ImF
two/multi-component gels provide the possibility to create LMWG have not been described before. The ImF can serve as
hybrid soft materials with diverse and tunable properties an excellent LMWG ligand to fabricate metallogels because it
through the noncovalent introduction of different functional possesses two N-atoms for metal-ion coordination and pro-
units within a single gel.30–38 Furthermore, the properties of vides rich non-covalent interactions such as hydrogen-bonding
the gels can be controlled by varying the molar ratios of the and π–π stacking interactions.27c The three molecular photo-
different components. Despite this, the preparation of two/ switches described in Fig. 1 below has been chosen because of
multi-component gels that contain metal centers remains their moderate solubility in water and ease of synthesis. The
largely unexplored.26b In particular, the photoresponsive gel- three compounds contain carboxylic acid groups covalently
to-sol phase transitions in two-component gels have rarely linked at different positions of the AzoPz moiety. The purpose
been extended to the study of metal–organic hybrid soft mate- of this strategy was to explore if the subtle structural changes
rials.26b This leaves scope to explore the photoswitchable be- in the molecular switches would lead to distinct coassembled
havior of metallogels suitably designed through the two/multi- gels. The presence of the carboxylic acid functional groups in
component approach, which is the goal of the present study.
the AzoPz units can induce noncovalent hydrogen-bonding
To date, azobenzene and its derivatives are the most widely interaction and provide metal-coordinating sites through the
used switchable molecules that have been integrated into N-pyrazole ligand which could facilitate the formation of
metallosupramolecular structures, host–guest coordination stable gels. Furthermore, the presence of the phenylalanine
cages, polymers and peptides for the development of photore- group from ImF provides the possibility of tuning the self-
sponsive self-assembly systems owing to their reversible trans- assembly through π–π stacking and by adjusting the pH and
to-cis isomerization in response to UV light.39–41 This photo- further enhances gelation ability.6,45 Owing to the presence of
induced isomerization results in a cis-isomer with a shorter the photoswitchable azo moiety, the resulting metallogels dis-
length scale and can lead to macroscopic change in the corres- played slow reversible light-induced gel-to-sol phase tran-
ponding materials. Despite this, azobenzenes also suffer from sitions upon alternating UV and green light irradiation. The
two major photoswitch performance issues: incomplete photo- light-responsiveness property and morphological features of
switching at a given wavelength of light and low thermal stabi- the metallogels could be tuned by varying the substitution pat-
lity of the cis isomer. In this context, arylazopyrazoles (AzoPzs) terns of the carboxylic acid functionalized AzoPz molecular
have been recently introduced as improved light-responsive switches.
molecular switches compared to their azobenzene counter-
parts.42 AzoPzs represent novel yet, largely unexplored emer-
ging molecular switches. AzoPzs have recently been found to
undergo better light-triggered reversible trans-to-cis isomeriza-
Results and discussion
Synthesis and characterization
tion compared to the commonly used azobenzene derivatives
by virtue of their heteroaromatic rings.42a–c Recently, the
Synthesis of AzoPz1–3. The small molecular azo switch
potential applications of these new molecular switches have derivatives, AzoPz1–3, were synthesized following standardized
been explored further for the development of several supramo- literature procedures.42a,f,44 Starting from commercially avail-
lecular systems ranging from host–guest complexes, foams, able aniline derivatives, AzoPz1–2 were synthesized in a simple
adhesives, DNA complexes, light-responsive peptide hydrogels three-step procedure (Scheme S1†). First, the 4-ethyl-amino-
with
host–guest
interactions
and
coordination benzoate derivative was converted into the diazonium salt by
complexes.42c,e,f,43 It is the aim of this article therefore to hydrochloric acid and sodium nitrite, which was subsequently
exploit these unique properties of AzoPzs for the development reacted in situ with 2,4-pentandione to afford the precursor
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Fig. 1 (a) Chemical structure of the ImF gelator; (b) molecular structures of the AzoPz-1, AzoPz-2 and AzoPz-3 photoswitches; (c) pictures of glass
vials containing the metallogel coassembly formed by the Zn2+ ion triggered self-assembly of ImF (MGC of 0.05 M) and the three photoswitchable
azo compounds at room temperature.
hydrazones as yellow solids in high yield (1a). The reaction of transform infrared (FTIR) spectroscopy. The 1H NMR spectrum
the resulting hydrazones with substituted hydrazines under of ImF shows peaks in the aromatic region ranging from 7.40
reflux conditions afforded the corresponding intermediate to 7.05 ppm and the aliphatic protons 3.77–2.94 ppm. The ImF
compounds (2a and 3a). Base hydrolysis of the resulting inter- compound was isolated as a white solid with good solubility in
mediate compounds afforded the desired AzoPz1–2 molecular alkaline aqueous solution.
switches in quantitative yield. Details of the synthesis and ana-
lysis of all the AzoPz1–2 are provided in the ESI, Scheme S1.†
Gelation study
AzoPz-3 can also be obtained by simple substitution and To test the gelation behavior of the imidazole substituted
hydrolysis reactions of the intermediate compounds (1b–3b) as phenylalanine based gelator (ImF), we first examined the self-
another series of azo switch (see the ESI, Scheme S2†). All the assembly of a series of metal ions such as Ca2+, Ni2+, Pb2+,
compounds have been characterized by 1H NMR, 13C NMR, Cu2+, Zn2+, and Mn2+ at room temperature (see the ESI,
and UV-vis spectroscopy. Similar to previously reported AzoPz Fig. S15†). The gelation studies were carried out by mixing an
based photoswitches, the presence of the characteristic peaks alkaline solution of ImF and aqueous metal salt solutions by
of the CH3 group of the pyrazole moieties in the vortex treatment. The results show that only Zn2+ induces the
2.50–2.60 ppm range in the NMR spectra indicates that, in all formation of a stable self-standing metallohydrogel, indicating
cases, the molecular switches exist in their more stable trans that the ImF gelator is Zn2+ selective. Upon mixing the two
isomers under normal conditions.42
components, the solution immediately turned turbid and
Synthesis of N-(1-H-imidazol-2-yl)methylidene-L-phenyl- transformed into an opaque white metallohydrogel after vortex
alanine (ImF). The synthesis of the imidazole (Im) containing treatment for 10 min which was confirmed by the test tube
the phenylalanine (Phe) derivative gelator, abbreviated as ImF, inverting method (Fig. 2). Meanwhile no gel formation was
is achieved by a condensation reaction between Phe and Im to observed without vortex treatment upon standing of the turbid
afford the intermediate imine product (1c) as shown in the mixture for several hours at room temperature. Application of
ESI, Scheme S3.† 27c Subsequent reduction of the imine inter- vortex treatment to the initial turbid mixture was found to be a
mediate product (1c) with NaBH4 at 0 °C afforded the corres- critical step in the self-assembly process and for the formation
ponding
N-(1-H-imidazol-4-yl)methylidene-L-phenylalanine of a stable metallohydrogel. It is also important to mention
(ImF) as a white solid in high yield. The resulting compound that the ImF gelator did not form any gel by itself under an
has been characterized by 1H NMR, 13C NMR and Fourier alkaline solution at pH ∼ 11.0. However, the combination of
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Fig. 2 Preparation of the Zn2+-triggered Zn–ImF metallohydrogel at the MGC of 0.07 M at room temperature.
the alkaline solution of ImF and aqueous solution of ZnCl2 pro- M when the molar ratio of ImF, Zn2+ and AzoPzs was 2 : 1 : 0.5
duced an opaque white metallohydrogel, indicating the critical at room temperature. It should be noted that before mixing,
role played by the Zn2+ ions in the gelation process. The effect the pH of the ImF solution was adjusted to pH ∼ 11.0 by using
of anions on gelation was further examined by using ZnSO4, Zn 1 M NaOH. Successful formation of the gels was confirmed by
(NO3)2, and Zn(OAc)2 under the same conditions. All zinc salts the inverted test tube method, which did not show fluid
formed white metallohydogels upon mixing alkaline solutions flowing down lasting for at least 30 min. All the investigated
of the ImF (pH ∼ 11.0) gelator with Zn2+ in a 2 : 1 molar ratio of carboxylic acid substituted AzoPz derivatives resulted in the
Zn2+ ions and ImF with a minimum gelation concentration of facile formation of a yellow colored metallogel with ImF in the
0.07M (see Table S1 and Fig. S14 in the ESI†). These results presence of Zn2+ ions. We also tested different mole ratios of
indicate that anions of the metal salt have no significant influ- Zn2+ and AzoPz to the ImF moiety following a similar pro-
ence on the formation of the gels. The formed metallohydrogels cedure, but no gels could be obtained.
were fairly stable, as no disruption of the gel was observed upon
standing for several months.
Transmission electron microscopy (TEM) study
To further investigate the morphological features of the metal-
logel coassembly obtained from Zn–ImF and Zn–ImF–AzoPzs,
transmission electron microscopy (TEM) images of the gels
Coassembly of ImF with Zn2+ and arylazopyrazole (AzoPz)
derivatives
The gelation properties of the coassembly system were tested were recorded. The TEM images of the different metallogels
in a manner similar to that of the Zn2+–ImF gels. As noted are shown in Fig. 4. These images reveal that the Zn–ImF gel
above, the ImF gelator alone or in combination with the AzoPz without the AzoPz photoswitches forms fine intertwined nano-
photo-switches could not self-assemble to form any gel even fibrillar network structures with an average diameter of ∼ca.
after vortex treatment. However, the combination of the alka- 30 nm and several micrometers (µm) long (Fig. 4a). The mor-
line solution of ImF with aqueous solution of Zn2+ salts and phology of the hybrid gels, formed from the Zn–ImF–AzoPzs’
AzoPzs dissolved in a minimum amount of dimethylsulfoxide coassembly (Fig. 4b–d), was similar to that of the Zn–ImF
(DMSO) resulted in the formation of stable yellow colored metallohydrogel, but it was also possible to discern some
metallogel coassemblies after vortex treatment (Fig. 3). The difference in terms of the length and diameter of the nano-
minimum gelator concentration (MGC) was found to be 0.05 fibers. The Zn–ImF–AzoPz-1 gel was comprised of a long
Fig. 3 Coassembly of Zn2+-triggered formation of the photoresponsive Zn–ImF–AzoPz metallogels at the MGC of 0.05 M at room temperature.
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Fig. 4 TEM images of metallogels obtained from (a) Zn–ImF (b) Zn–ImF–AzoPz-1 (c) Zn–ImF–AzoPz-2 (d) Zn–ImF–AzoPz-3 in an alkaline solution
of pH ∼ 11 : 00.
entangled network of nanofiber structures with an average dia- The TEM images demonstrate that the morphology of the coas-
meter of ∼ca. 60 nm (Fig. 4b). The TEM image of the Zn–Im– sembled gels could be tuned by changing the structure of the
AzoPz-2 coassembly showed a dense network of nanofibers non-gelling AzoPz units. It is also hypothesized that the Zn2+
whose diameters are ∼ca. 80 nm wide (Fig. 4c). Notably, the ions cross links and interconnect the gel aggregates to afford
structure of the Zn–Im–AzoPz-3 coassembly was a thin den- the supramolecular hybrid gels.27a
drite like morphology, which is different from those of the
nanofiber structures obtained from the Zn–ImF and the Zn–
Rheological study
ImF–AzoPz-1 and Zn–ImF–AzoPz-2 coassemblies (Fig. 4d). The
inner diameter of these dendrite structures was ∼ca. 10 nm.
We speculate that the presence of an additional hydroxyl func-
tional group attached to the N-position of the pyrazole ring in
the AzoPz-3 unit (see the molecular structure in Fig. 1) might
be responsible for the formation of loose dendrite like struc-
tures. The hydroxyl group may provide additional opportunity
for hydrogen-bonding interaction enhancing supramolecular
cross-linking in the self-assembly process and gel formation.
The viscoelastic properties of the metallogel coassemblies
formed from Zn–ImF and Zn–ImF–AzoPzs have been investi-
gated by rheological studies at room temperature using freshly
prepared samples. The dynamic amplitude sweep rheological
experiments conducted on the Zn–ImF and Zn–ImF–AzoPz
coassemblies showed that the storage modulus (G′) of all the
gels are found to be significantly higher than their loss
modulus (G″), indicating that the gels are elastic rather than
viscous materials (Fig. 5a). Comparative changes in the G′/G″
Fig. 5 (a) Dynamic strain amplitude rheological experiment for the Zn–ImF and Zn–ImF–AzoPzs gels; (b) frequency sweep rheological experiment
for the Zn–ImF and Zn–ImF–AzoPzs gels.
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values also clearly show Zn–ImF and Zn–ImF–AzoPz-1 with the exposing the sample with UV light (λ = 365 nm), the strong
strongest viscoelastic character in these series. Meanwhile, the absorption band at 341 nm corresponding to the π–π* tran-
G′ and G″ values of the Zn–ImF–AzoPz-2 and Zn–ImF–AzoPz-3 sition is significantly reduced and blueshifted, whereas the
gels were lower than that of the Zn–Im gel-i.e., the AzoPzs absorbance around 435 nm due to the n–π* transition has
make the gel network less stiff. Such changes in the visco- increased in intensity and shows a slight redshift (Fig. 6a).
elastic properties may be attributed to the slight structural Upon irradiation with green light (λ = 530 nm), the original
variation of the AzoPz molecular photo-switches, which is in spectrum for the trans-isomer is restored showing a strong
agreement with the TEM images. In the frequency sweep absorption band with a maximum centered at 341 nm and a
experiment (Fig. 5b), all the metallogel coassemblies were also smaller band at 435 nm (Fig. 6b).42a,b These considerable
found to display a higher storage modulus (G′) than the loss changes in the absorption spectra are indicative of the
modulus (G″) which is in agreement with the dynamic ampli- efficient reversible trans-to-cis light-induced isomerization of
tude sweep. These observations suggest the formation of visco- the AzoPz-1 moiety under alternating UV and green light
elastic metallogels that are fully stable in the detection range irradiation. The photostationary state (PSS) was reached in 80 s
of the frequency sweep. In addition, the G′ and G″ values also and the cis : trans ratio in the PSS was found to be 88 : 12 for
clearly indicated that Zn–ImF–AzoPz-3 has much reduced AzoPz-1 as determined by UV-vis spectroscopy.
stiffness compared to those of the Zn–ImF and the Zn–ImF–
The photoisomerization behavior of the AzoPz-2 and AzoPz-
AzoPz1-2 gels. Overall, the results imply that the metallogels 3 molecular switches in solution was examined in a manner
are fairly tolerant to external force.
similar to that of AzoPz-1 by alternating UV and green light
irradiation (Fig. S19 and S20†). In general, both AzoPz-2 and
AzoPz-3 also showed similar reversible photoisomerization be-
havior to that of AzoPz-1. Irradiation with 365 nm light leads
to a decrease in intensity of the π–π* absorbance, whereas the
intensity of the n–π* absorbance increases, which is character-
istic of isomerization from the thermodynamically stable trans-
isomer to the cis-isomer. The back-switching was examined
after irradiation with green light. The intensity of the ∼345 nm
maximum absorption band was fully regained with the orig-
inal spectrum being recovered with a minimum of 85% of the
original intensity, confirming the efficient reversibility of the
Photoisomerization properties of the AzoPzs in solution
The photoswitching properties of the three molecular switches
(AzoPz1–3) functionalized with carboxylic acid substituents
were studied by using UV-visible spectroscopy. As a representa-
tive example, the light-induced photoswitching behavior of
AzoPz-1 is shown in Fig. 6a. Before light irradiation, the solu-
tion of AzoPz-1 exhibited the characteristic strong absorption
band at 341 nm corresponding to the π–π* transition of the
trans-isomer of the AzoPz-1 moiety, whereas the n–π* absorp-
tion peak appeared as a relatively weak band at 435 nm.42 After
Fig. 6 UV-vis absorption spectra of AzoPz-1: (a) trans-to-cis upon irradiation at λ = 365 nm 0–80 s. (b) cis-to-trans isomerization green light (λ =
530 nm) 5–180 min.
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photochromic process (Fig. S19 and S20 in the ESI†). At the molecular interactions resulting in gel-to-sol phase transitions
PSS, AzoPz-2 showed 89% for trans-to-cis and 94% for cis-to- of the Zn–ImF–AzoPz metallogels. An analysis of our results
trans. On the other hand, AzoPz-3 showed 90% for trans-to-cis showed that the gels formed from the Zn–ImF–AzoPz coassem-
and 87% for cis-to-trans.
bly exhibit slow photo-induced reversible gel-to-sol phase tran-
sitions. As a representative example the photochemical phase
transition of the Zn–ImF–AzoPz-1 metallogel coassembly is
shown in Fig. 7. Upon irradiation with UV light (λ = 365 nm)
the gel formed from the Zn–ImF–AzoPz-1 coassembly began to
collapse, inducing a gradual gel-to-sol transition (2 h). A com-
plete conversion of the gel into the solution took about 4 h
(Fig. 5a). The resulting solution could be changed into the gel
sate upon green light irradiation at λ = 530 nm for 15 min fol-
lowed by vortex treatment. The metallogel could also be re-
formed by letting the sample rest at room light for 5 h and
vortex treatment. This phase transition could be attributed to
the photoisomerization of the AzoPz-1 unit from the planar
trans-isomer to the twisted cis-isomer (Scheme 1), disrupting
the gel self-assembly, presumably by destroying the nanofiber
organization and π–π stacking. Previous studies by Ravoo and
co-workers have shown that the loss of planarity in the cis-con-
formation of the AzoPz containing gels leads to less favorable
intermolecular interaction resulting in the formation of a solu-
tion.42e Upon green light irradiation (λ = 530 nm), the planar
trans conformation is restored and the metallogel is re-formed
after vortex treatment as confirmed by the inverted vial test
(Fig. 7a). In addition, the gel-to-sol phase transition is
accompanied by a color change from yellow to orange, which
Photoresponsive properties of metallogel coassemblies
Based on previous studies on photochromic AzoPz containing
gels, we hypothesize that the AzoPz units would contribute to
the supramolecular interactions responsible for the formation
of the entangled nanofibers imaged by TEM through inter-
molecular π–π stacking, and van der Waals interactions.27c,42e
In this context, we supposed that the photo-induced trans-to-
cis isomerization of the AzoPzs should have influence on the
self-assembly system through the disturbance of the inter-
Scheme 1 Reversible trans-to-cis photoisomerization of AzoPz-1.
Fig. 7 (a) Photographic image of the reversible photo-induced gel-to-sol phase change of the gel formed by the Zn–Im–AzoPz-1 coassembly
upon alternating UV and green light irradiation; (b) changes in the UV-Vis spectrum of Zn–Im–AzoPz-1 upon irradiation λ = 365 nm and recovery of
the spectrum after irradiation with green light λ = 530 nm.
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could be attributed to the isomerization of the AzoPz moiety partial gel-to-sol phase transition to occur. The overall long
from the trans to the cis-isomer. irradiation time needed for the gel-to-sol phase transition to
The gel-to-sol switching process was further confirmed by occur could be due to the opaque nature of the multi-com-
monitoring the UV-vis spectral changes of a DMSO solution of ponent metallogels. These results indicate that the gel-to-sol
Zn–ImF–AzoPz-1. Before UV light (λ = 365 nm) irradiation, the phase transition slightly depended on the chemical structure
Zn–ImF–AzoPz-1 gel exhibits a strong absorption band at of the AzoPz molecular photo-switches.
341 nm, arising from the π–π* transition of the trans-form of
Fourier-transform infrared spectroscopy (FT-IR) studies
the AzoPz-1 chromophore (Fig. 7b). Meanwhile, the n–π*
absorption of the AzoPz-1 moiety appeared as a relatively weak
band at 440 nm. After exposing a solution of the metallogel
coassembly to UV irradiation for 4 min, the absorption band
at 340 nm decreased and new absorption bands appeared at
290 nm and 450 nm corresponding to the cis-isomer. The light
irradiation also caused a blueshift of the π–π* transition and
redshift of the n–π* transition. Upon green light irradiation for
15 min, the 345 nm absorption band of the trans isomer of the
AzoPz-1 unit again appeared, and the 290 and 450 nm signals
of the cis isomer disappeared. Thus, the absorption spectrum
of the gel re-produced by green light irradiation, which was
identical to that of the initial gel state. These results indicate
that the changes in the absorbance intensities of the Zn–ImF–
AzoPz-1 coassembly are induced by the trans-to-cis isomeriza-
tion of the AzoPz moiety. The conversion efficiency of cis-
AzoPz-1 can reach ca. 80% in the photostationary state within
15 min estimated based on the UV-vis spectra. Additionally,
there was no significant change in temperature of the gel
during the light-irradiation process. When the orange solution
formed by the UV irradiation was stored for 24 h in the dark,
the sol state was maintained with no noticeable gel formation.
This indicates that the observed gel-to-sol phase transitions
proceeded by light-induced photoisomerization.
IR spectra were recorded for the pure ImF, the Zn–ImF and the
Zn–ImF–AzoPz metallogels (Fig. S21 and S23 in the ESI†). The
FTIR spectrum of the pure ImF in the non-gel sate showed
peaks at 3087 cm−1 (–COOH), 3387 cm−1 (–NH) and 1609 cm−1
for the imidazole ring. Meanwhile, for the Zn–ImF gel, the
peak at 3087 cm−1 (–COOH) showed a much reduced intensity
and the peak at 3387 cm−1 (–NH) shifted to 3402 cm−1
,
suggesting that both the carboxyl O and amino N atoms of the
ImF gelator are forming a coordination bond with the Zn2+
ion. In addition, the enhanced intensity of the N–H band at
3400 cm−1 in the FTIR of the Zn–ImF gel may suggest the pres-
ence of hydrogen-bonding interaction that may arise from co-
ordinated water molecules in the self-assembly. Moreover, the
1609 cm−1 peak of the imidazole (–NH) stretching shifted to
1600 cm−1 indicating the involvement of the imidazole N–H
moiety in hydrogen-bonding. The FTIR spectra of the Zn–ImF–
AzoPzs were almost identical to that of the Zn–ImF gel except
for the presence of additional aliphatic region stretching fre-
quencies (Fig. S21 and S23 in the ESI†). These results indicate
that the incorporation of the small molecular switches into the
structure of the Zn–ImF assembly did not affect the coordi-
nation bond that existed between Zn2+ ions and the ImF
ligand.
The Zn–ImF–AzoPz-2 and Zn–ImF–AzoPz-3 gels exhibit
similar spectral changes to that of AzoPz-1, but with a slight
lower conversion efficiency (see Fig. S19a, b and S20 in the
ESI†). Prolonged irradiation with UV light (λ = 365 nm)
induces a partial gel-to-sol phase transition (Fig. S20†). It took
9 h for Zn–ImF–AzoPz-2 and 12 h for Zn–ImF–AzoPz-3 for the
X-ray diffraction (XRD) studies
To further provide evidence for the self-assembly process of
the different samples, X-ray diffraction studies were performed
using vacuum dried xerogels. The XRD patterns of the nano-
structured samples are shown in Fig. 8. The XRD results indi-
Fig. 8 XRD pattern of the Zn–ImF and Zn–ImF–AzoPz coassembly xerogels.
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cated that the samples presented an ordered structure. For the widths and lengths. Owing to the presence of the AzoPz photo-
Zn–ImF sample, the presence of a prominent characteristic switchable components, the metallogel assemblies are able to
peak at a d-spacing of 3.63 Å suggested typical π–π stacking form photoresponsive gels that display a light-triggered phase
interactions in the gelling process. In addition, another peak transition. The metallogel coassemblies undergo a slow revers-
with a d-spacing of 2.0 Å can be assigned to H-bonds among ible light-triggered gel-to-sol phase transition through the
the Zn–ImF gel network.27,46,47
trans-to-cis isomerization of the AzoPz moiety under alternat-
Furthermore, the XRD of the hybrid gels was studied using ing UV and green light irradiation, resulting in partial or full
Zn–ImF–AzoPz-3 as a representative example. The results indi- disassembly of the gels. The reversible gel-to-sol photomodula-
cated that the addition of the AzoPzs did not significantly tion of the metallogel coassemblies was achieved through the
change the self-assembly mode of the Zn–ImF metallogel. The non-covalent introduction of the carboxylic acid functionalized
XRD pattern of the xerogel of the Zn–ImF–AzoPz-3 showed a AzoPz molecular switches. The gel-to-sol phase transition of
series of peaks at the same position as in the main component the metallogels was also found to be slightly dependent on the
Zn–ImF gel (Fig. 8). It may be possible that the hybrid gels are chemical structure of the AzoPzs; AzoPz-2 and AzoPz-3 deriva-
composed of mainly Zn–ImF fiber networks extended by the tives took a longer time than AzoPz-1. The overall long
additional AzoPzs as observed in the TEM images.
irradiation time needed for the gel-to-sol phase transition to
occur could be due to the opaque nature of the multi-com-
ponent metallogels. This work provides some inspiration for a
convenient fabrication of a new class of photoresponsive coas-
sembled soft materials that combine metal specific properties
and light.
Mass spectra [MS (ESI) studies
As described above, the ligand : metal ratio that gives the most
stable gel is 2 : 1. This was corroborated by a MS (ESI) study, in
which the presence of the 2 : 1 ratio was identified by the for-
mation of an ionized form of the Zn–ImF complex, [(ImF)2Zn]+
(see the ESI, Fig. S24†). Based on all the above observations,
we speculate that ImF likely acts as a tridentate ligand, which
binds to Zn2+ and forms a hexa-coordination compound of
Zn–ImF. The initially formed Zn–ImF metal coordination
complex may first self-assemble into fibers and further cross-
link the fibers to form intertwined nanofibrillar networks
through various non-covalent interactions that include
H-bonding interactions and π–π stacking interactions.
However, due to the dynamic nature of the coassemblies,
attempts to get a clean MS spectra of the hybrid gels were not
successful. Spraying a dilute methanol solution of the hybrid
gel showed the presence of prominent peaks that belonged to
[(ImF)2Zn]+ (data not given).
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
We thank the U.S. National Science Foundation (NSF) for
financial support through the Partnership for Research and
Education in Materials (PREM) (NSF PREM: Award # 1827820)
grant. We thank Dr. Silvina Pagola and Isaiah Ruhl of Old
Dominion University for XRD and mass spectroscopy data col-
lection and analysis, respectively. We also thank Dr. Helge
Heinrich of the University of Virginia for the collection and
analysis of TEM data.
Conclusions
In summary, we have described a simple two/multi-component
approach to construct hybrid photoresponsive zinc-based
metallogel coassemblies. These gels are formed by mixing an
alkaline aqueous solution of an imidazole substituted phenyl-
alanine derivative gelator (ImF) with carboxylic acid functiona-
lized arylazopyrazole (AzoPz) molecular switches and aqueous
solutions of zinc salts. We found that all of the AzoPzs could
form metallogels with an aqueous solution of the ImF gelator
in the presence of Zn2+ ions and self-assemble into nanofibers
with different morphologies. However, the ImF alone or mixed
with the AzoPz photoswitches does not form any gel. This
observation indicates that the gelation process is triggered by
the Zn2+ ions. The results also showed that the gel formation
is specific to Zn2+ ions. Furthermore, the viscoelastic behavior
of the resulting metallogels could be modulated by a structural
variation of the small molecular photoswitches. Depending on
the functional group linkage of the small molecular switches,
the self-assemblies gave nanostructures with slightly different
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